This invention relates generally to well pumping units and more particularly to an improved safety mechanism for shutting down operation of the pumping unit in the event of failure of one or more components on the lift side of the pumping unit, when under load. Such failures, although rare, have disastrous consequences both for personnel in the area and the equipment being used.
The present invention has utility with a wide variety of well pumping units. One such well pumping unit includes a tower or mast mounted on a base platform, a source of power and a winding drum on the base platform, a lift belt made of conveyor belting from the winding drum up the tower and over a crown spool mounted atop the tower and then extended downwardly and connected to the polish rod of a well pump, and a reversing mechanism associated with the power means reciprocate the belt and thus the polish rod, and thereby to operate the pump. A counterweight or weight box is interposed in that portion of the drive belt between the spool and the winding drum so that power requirements of the pumping unit are kept to a minimum. An idler spool is provided at the base of the tower and that portion of the lift belt between the counterweight and the winding drum is trained beneath the idler spool so as to restrict movement of the counterweight to a generally vertical direction within the tower during operation of the pump.
The need for a safety mechanism is particularly acute during a lifting stroke of the pumping unit. The polish rod load may well be in the area of, for example, 30,000 pounds and the counterweight will be weighted only somewhat less than the polish rod load. If that portion of the lift belt between the spool on the top platform and the polish rod should fail or if one or more of the polish rod, rod string and sucker rod components of the well pump should fail, the counterweight will fall to the base platform and the lift belt will unravel from the spool; the possible, disastrous consequences are self-evident. Accordingly, this invention provides a mechanism for immediately arresting and locking the counterweight in place in the event of a failure as just described.
A brief description of the development of well pumping units is in order. In the early life of a well, reservoir pressure alone may be sufficient to raise the oil to the surface, providing local regulatory authorities permit such a procedure. However, such pressure is eventually exhausted and the oil must be pumped to the surface to be recovered. The most common variety of pump employed for this purpose is a walking beam pump having a nominal stroke distance of from about seven to twelve feet. A walking beam pump is suitable for shallow to medium depth wells, but such a pump becomes inefficient as stroke frequency increases. Specifically, rod stretch, dynamics and pump volumetric efficiency combine to decrease efficiency as stroke frequency increases.
Thus, long stroke well pumping units, particularly useful in deep wells, have been developed, some of which have stroke lengths of thirty-two feet or more. One example of such a prior art long stroke pumping unit is the "Oilwell" Model 3534 Long Stroke Pumping Unit, manufactured by Oilwell, a division of United States Steel. The unit includes a central tower having multiple braces to stabilize the structure, a complex multi-strand cable crown block assembly suspending the rod string and a variable capacity counterweight and, of course, a prime mover. Several safety systems are provided, including an automatic air brake system controlled by an overspeed governor flyweight and actuated when the counterweight exceeds a predetermined, acceptable downward speed. Other safety features include interlocked controls and automatic breaking in the event of an air loss or power failure. Both the pumping unit and the safety features provided are complicated and quite expensive.
Another example is prior U.S. Pat. No. 3,248,958 issued to Emil A. Bender, which discloses and claims a wire line deep well pumping apparatus and a safety brake system which includes a somewhat complex system for jamming a cam against the wire lines in sheaves mounted atop a tower in the event of rod string failure, thus preventing the counterweight from falling. Another prior U.S. Pat. No. 3,483,828 issued to Emil A. Bender, also discloses a deep well pumping unit, and describes generally a braking system which is actuated in the event of failure. A more recent example in a long stroke pumping unit is yet another invention of Emil A. Bender which is the subject of a co-pending application Ser. No. 393,102, filed June 28, 1982, and licensed to the Assignee of the present invention, Baker Pro-Lift, Inc. The fail safe mechanism described in that application is arranged in a lift belt system and includes a wedge and brake shoe combination mounted on the top of the tower which is actuated to grasp the lift belt in the event of fracture of the belt, polish rod, rod string or sucker rod on the polish rod side of the crown spool.
However, the prior art does not disclose a safety mechanism for well pumping units of the type described herein which provides sensitivity and consistent reliability over the wide range of load fluctuations that may be experienced within even a single cycle of the pumping unit. The load on the polish rod side of a lift belt system varies in a representative installation from a maximum of approximately 29,700 pounds, when the polish rod is at its lowermost position and beginning an upstroke and thus experiencing the full fluid and dynamic load, to a minimum of 18,100 pounds, when the polish rod is at its uppermost position and beginning to fall back down through the fluid. The counterweight or weight box in this example is loaded with 16,300 pounds or approximately 90% of the minimum load on the polish rod side, to reduce power requirements of the system to a minimum and yet provide sufficient weight differential to allow the polish rod and rod string to fall gently through fluid to its lowermost position again. The prior art safety mechanisms all sense off of the polish rod side of the system and therefore must withstand the maximum load to minimum load fluctuations which requires a heavy duty safety mechanism and an attendant sacrifice of sensitivity. Attempts to increase sensitivity in such prior art examples results in frequent malfunction wherein the brake or safety mechanism prematurely locks up and disrupts the operation of the pumping unit although no failure or fracture in the lift belt system has occurred.
Reference to other arts where a similar problem is experienced leads inevitably to the elevator art and in particular to prior emergency brake systems developed therein to avert human tragedy in the event of cable separation. It was recognized in the very early stages of this art that a rack and pawl combination was the most dependable safety latch and the risk to human life demanded a heavy duty arrangement. Representative examples of such prior art safety devices include U.S. Pat. No. 931,211, issued to E. E. Moulton, wherein a cross bar at the top of an elevator cage is secured to the cable and counter-biased by a coiled spring against cable tension. Tension in the cable overcomes the counter-bias and the cross bar engages and pivots a pawl arranged at either side of the elevator cage out of engagement with corresponding racks mounted in alignment therewith in the elevator shaft. The pawls are spring-biased toward a position in engagement with the rack, however, so that upon cable failure and the attendant release of tension therein, the counter-bias of the coiled spring forces the cross bar out of engagement with the pawls and allows the pawls to spring into engagement with the corresponding racks to arrest free fall of the elevator cage. A variation of this concept may be seen in U.S. Pat. No. 1,482,331, issued to J. Vanslett, wherein the pawls are spring-biased out of engagement with the corresponding racks, and the counter-bias force (provided in this case by a pair of opposing leaf springs) drives a wedge arranged between the pawls to engage the pawls with the corresponding racks in the event of cable failure. In another such example, disclosed in U.S. Pat. No. 1,302,059, issued to J. A. Linn, the safety devise is mounted beneath the elevator cage and connected to the cable above the cage through an inverted U-shaped yoke arranged to pull a pair of pawls in scissors fashion out of engagement with corresponding rack when the cable is in tension, and counter-bias springs force the pawls again in scissors fashion into engagement with the racks upon cable failure.
One obvious distinction in the foregoing and similar examples of prior safety mechanisms in the elevator art lies in the fact that the lift cable or cables are exclusively tensioned above the elevator cage; thus, each of the various safety mechanisms conceived to respond to cable failure must of necessity have an operative link to the cable above the cage for actuation. Another significant distinction between such examples and the lift belt, well pumping units previously described is that the maximum load and load fluctuations experienced in normal duty is significantly less. Therefore, the responsiveness and sensitivity are not compromised by the mass of the system or extreme load differentials and the attendant sizing difficulties for actuating mechanisms such as the springs described. The foregoing distinctions are best put in perspective by considering the complexity and near impossibility of sizing actuating mechanisms such as the springs described for heavy duty service wherein loads approaching 30,000 pounds and load fluctuation of 12,000 pounds, or more, are experienced under normal operating conditions, which actuating mechanisms must be sensitive to load failure and yet avoid premature actuation due to normal load fluctuations.
Accordingly, the prior art does not disclose a safety mechanism for well pumping units and the like which provides sensitivity and consistent reliability over a wide range of severe loads and load fluctuations without premature actuation, and which is of uncomplicated structure and requires no power means in order to be operated. Additionally, the prior art does not disclose the safety mechanism herein disclosed and claimed in a lift belt system for transmitting reciprocating motion to the polish rod in a long stroke, well pumping unit.